Don't ever get cocky. It'll only make you work less and disappointed when results come out (coupled with the fact that cocky people are usually quite arrogant, which is never a good quality to have in the first place!)
Perhaps this is what was the flaw of your friends who thought they were guaranteed 45+? I guess people should never really underestimate the difficulty to get such a score. Even if the subject is considered "easy", VCE is the whole of victoria not just a small bunch of people so it's gonna be competitive no matter what. Some subjects can be quite deceiving as the concepts can be easy or quite understandable although the subject is then usually marked more harshly to help distinguish the As and A+s from the Bs and C+s (cough psych cough), so be careful with ones like that.
I think a lot of people fall into a false sense of security with sacs (I'm talking more about lower level schools, as I witnessed this at my own school), I know a lot of my friends thought "Yay I'm doing well in sacs, I'm good at this subject...hmm I probably don't need to study too much for the exam then." I know I personally never relied on my sacs, I tried to block my sac scores out completely. I knew the exams were the main determinant of my scores so I focused most of my effort into preparing for them.
Getting high scores is achievable through hard work, knowing your course work well, doing practice exams, discovering certain tips and tricks that are specific to the subject you're doing, and also a bit of luck.
I guess you're lucky in a way that you have two mid year exams so you'll at least be ahead of those who didn't do a 3/4 in year 11 and have no mid years. And you'll be able to judge where you stand when you get your mid year results, so that'll also help for end of years.
I wish you all the best of luck!